Agronomy: Dealing with salty soil


By Digital team | 11 January 2018
soil
Photo: Lucille Botha

Too much salt in water and soil can have a dire effect on crops, hampering germination and growth. 

There are a number of causes for salty soil. Some soil is intrinsically saline, and poor drainage, climatic trends that lead to the accumulation of salt in soil, and human activities such as land clearing can worsen the issue.

Salinity is especially prevalent in arid and semi-arid regions, and can lead to the corrosion of roads, bricks, pipes and cables. It also reduces water quality.

MANAGEMENT OPTIONS

In their report Effect of Irrigation Water and Water Table Salinity on the Growth and Water Use of Selected Crops, South African Water Research Commission researchers propose 4 management options to get rid of excess salt:

  • Irrigate more than the expected crop water use.
  • Irrigate the same amount as the expected water use. Less excess salt will be washed out, but at the same time less will be added to the root zone.
  • Irrigate less than the expected crop water use. (Take care that the reduction does not exceed the expected water-table uptake of the salinity, which could increase the rate of salination in the capillary layer.)
  • Revert to salt-tolerant crops such as red beets, spinach, barley, cotton, and some grasses used for animal feed, such as wild rye and wheat grass. Crops that have a low tolerance for salinity include peas, beans, maize, wheat, some fruits, and a number of clovers used for animal grazing.

REMOVING EXCESS SALT

According to the United Nations Food and Agricultural Organisation (FAO), excess salt can be removed through the use of chemicals and engineering. Possible engineering solutions include:

Scraping – accumulated salt on the surface is removed by mechanical means. Success is limited and will only temporarily improve crop growth.

Flushing – small amounts of accumulated salt on the surface are washed away by flushing water over
the surface.

Leaching
– fresh water is “ponded” on the soil surface and allowed to infiltrate, washing away salty water through subsurface drains. Leaching should preferably be done when the soil moisture content is low and the groundwater table is deep.

Factors like the salt content of the soil, the desired level of salinity and soil characteristics will determine the amount of water needed for desalination. A useful rule of thumb is that a unit depth of water will remove nearly 80% of salts from a unit soil depth.

Therefore, 30 cm of water that pass through the soil will remove approximately 80% of the salts present in the upper 30 cm of soil. For more reliable estimates, conduct salt-leaching tests on a limited area.

Drainage
– according to the FAO, drainage serves two purposes – to reduce waterlogging, and to control and reduce salination. Proper drainage is essential to allow crop diversification and intensification, the growth of high-yielding varieties, effective use of inputs such as fertilisers, and mechanisation.

Drainage can be improved through land-forming techniques, surface-drainage structures and management of soil structure through improved tillage and organic-matter management. Land-forming techniques can be described as man-made interventions to slope agricultural land for irrigation.

Surface-drainage structures include bedded and graded systems. Improved tillage refers to the preparation of the soil by ploughing, ripping, or turning it, while organic-matter management is essential to ensure that crops thrive.

Most soil and water amendments in common use supply calcium directly or indirectly through an acid or acid-forming substance, which reacts with the soil lime to release calcium. The simplest and most cost-effective method to get rid of excess salts, however, is by leaching it with irrigation water, says Dr. Piet Nell from the South African Agricultural Research Council’s (ARC) Institute for Soil, Climate and Water.

MONITORING SYSTEM

Nell says it’s extremely important to have a sound soil and water monitoring system in place. If you decide to have your water tested and monitored scientifically, do your homework before you appoint someone to do the analyses and interpretation for you.

SOME EXPERT ADVICE

  • The majority of crops are sensitive to salt during the germination and early growth phases. Move your plant date to after a bout of rain – rainwater will reduce the salt content, improving the quality of water.
  • Ridge your fields and use a micro-drip rather than overhead irrigation systems
  • Too much salt can burn the leaves of crops or fruit. Take note of when these burns appear, and plan your planting season and irrigation schedules accordingly.
  • Water analysis should be done at least twice a year – in spring and in autumn. Regular testing is advised because water quality differs from time to time.
  • Invest in field electrical conductivity (EC), total dissolved solids (TDS), pH and nitrate meters.
    EC and TDS meters measure the electrical conductivity or salt content of water, whereas a pH meter is used to measure the acidity or alkalinity of water.
  • A nitrate meter allows farmers who make use of fertilisers to accurately determine the level of nitrate in water, and when and how much is required, thus minimising the possibility of wasted fertiliser.
  • These meters are available as hand-held and battery-operated.
  • Different crops have different tolerances for salt, after which a decline in yields occur.
  • Too little salt in water can also cause problems. Water becomes corrosive then, and can potentially destroy your irrigation system.
  • Salt can also destroy cement channels, and iron and manganese can clog your irrigation system.

Also read:
Agronomy: Planting tips for wet soils
Agronomy: How to look after wetlands on your farm

  • This article was written by Rene Bosman and first appeared in Farming SA.